Considerable interest has been shown in recent years for static dissipative and electrically conductive laminates for use in various environments, including static dissipative work surfaces and conductive flooring materials. Among the recent patents there may be mentioned the U.S. patents to Wilks et al U.S. Pat. No. 3,922,383; Grosheim et al U.S. Pat. No. 4,472,474; Cannady et al U.S. Pat. No. 4,480,001; Cannady U.S. Pat. No. 4,540,624; Berbeco U.S. Pat. No. 4,454,199 and Berbeco U.S. Pat. No. 4,455,350. The use of carbon filled paper is known, as is the use of salts, the latter having been previously known, noting patents such as Meiser U.S. Pat. No. 3,650,821 and Economy et al U.S. Pat. No. 3,567,689. However, no one product is suitable for all static dissipative and conductive environments, because different usages, i.e. environments, require different properties.
The Department of Defense defines the following relationship between static electrical properties and surface resistivity (in ohms/square):
anti-static: greater than 10.sup.9 PA1 static dissipative: between 10.sup.6 and 10.sup.9 PA1 conductive: less than 10.sup.6.
Surface resistivity of standard high pressure decorative laminate is about 10.sup.11 to 10.sup.13 ohms/square.
A static dissipative laminate having a resistivity on the order of about 10.sup.6 to 10.sup.9 ohms/square is needed for a work surface for the assembly of electronic components. Losses of electronic components attributable to electrostatic discharge amounts to tens of millions of dollars each year. Typically a tray or tote bin of electronic components such as integrated circuit chips, in being moved around, picks up a charge of thousands of volts which can reach 30,000 volts when the air is dry. When a so charged component is touched or put down, the charge is suddenly discharged, destroying or damaging the component. For this reason, electronic components are packaged in conductive containers. Workers wear special conductive clothing and shoes. Employees are grounded via wrist bands and floor mats. Air is conditioned and ionized. An assembler, who's hand is kept at zero potential by a grounded wrist strap, can "zap" a chip when he picks it up. If the table top, however, upon which the tote bin is placed is static dissipative and connected to a ground, then the 30,000 volts of charge will leak off the components before the operator touches them.
Thus, an important link in the chain of protection for the components against electrostatic discharge during assembly, repair and use is the work surface on which assembly takes place, i.e. the work top. It is important that this work top not be conductive. As noted above, its resistance should be about 10.sup.6 to 10.sup.9 ohms, and all work tops of less than 10.sup.6 ohms resistance are grounded through a 10.sup.6 ohm resistor. If the work top has a resistance of less than 10.sup.6 ohms, it becomes a safety hazard for electrical shock, a path for current between components, and it discharges too rapidly which may result in damage to components.
Present static dissipative laminates may suffer from other disadvantages in addition to being either too conductive or not conductive enough. Thus, some presently available static dissipative laminates have an upper surface containing carbon particles for providing a conductive path from the upper surface of the laminate to the interior. This can result in dusting of conductive material from the surface of the laminate as it wears, which conductive material by itself will result in damage due to electrical short circuits. In addition, the color of these laminates is limited to black, which can provide human engineering problems.
Another problem which occurs with conventional static dissipative laminates is that the surface of the laminate tends to lose its electrical conductivity when the relative humidity drops in winter time. Measured resistivity of conventional static dissipative and conductive laminates is strongly dependent on relative humidity, and can change several orders of magnitude between 50% relative humidity and 15% relative humidity. Prior art static dissipative and conductive laminates do not perform well at relative humidities below 25-30%. For this reason, work areas may have to be humidified, which is not always desirable due to the possibility of inducing corrosion in certain products and in certain equipment as well. In addition, the necessity for precise humidification increases the cost of handling the electronic components.
A number of high pressure decorative laminates having static dissipating or conducting properties are already on the market. Two of these use a highly conductive carbon impregnated layer below the decor sheet. Of these, one has an excessive surface resistivity and it appears that the upper layer is not sufficiently conductive. The other uses quaternary ammonium compounds in the upper layer, along with the conductive carbon containing paper therebelow, and while this laminate is adequate at normal relative humidity (about 50%), it is inadequate at low relative humidities. A third product of yet another manufacturer, although somewhat better, is still inadequate at low relative humidities.
Conventional static dissipative laminate has also introduced the problem of field suppression. This occurs when the laminate is constructed of a highly conductive layer buried under a relatively non-conducting surface. When a charged object is placed on the laminate surface, a field is induced in the buried conductive layer forming what is, in effect, a leaky capacitor. The overall result is that to an outside observer, e.g. a static electricity sensing meter such as an electrometer, a zero electrical potential exists when, in reality, the field is hidden within the laminate. When an object such as an electronic component is lifted from the laminate surface, the charge reappears thereby creating the static electricity hazard sought to be avoided.